Gang Hero

As stands, the variety of published Mobs is small, but not quite as small as might seem:

Gorkers.
Morkers.
Freebooterz. [Also do-able as a Gorker mob, for boarding actions.]
Bikerz. [Also do-able as a Morker mob, with so many drivers.]

Then additionally Grotz, Muties, and Diggaz.

Thematically, the various Paul Bonner era clans ['Ere We Go! and Waaaargh! books] can be pretty easily represented through the Gorker/Morker split, just by choosing equipment and modeling/painting style as the mob goes on [painting red vehicles and getting them souped up, or painting blue faces with a lot of snazzguns], except for Snakebites. Yes, over the years lots of people have home-brewed up GorkaMorka Snakebitez rules, but the best of them look to be Mutie Raiders with sticks and bows and slings. Thematically, I think you could just use a bunch of cyboars as "counts as bikes" and make as good a go of it.